Memory devices are typically provided as internal storage areas in computers. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed on an individual basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge on the floating gate.
NOR and NAND flash memory devices are two common types of flash memory devices, so called for the logical form the basic memory cell configuration in which each is arranged. Typically, for NOR flash memory devices, the control gate of each memory cell of a row of the array is connected to a word-select line, and the drain region of each memory cell of a column of the array is connected to a bit line. The memory array for NOR flash memory devices is accessed by a row decoder activating a row of floating gate memory cells by selecting the word-select line coupled to their gates. The row of selected memory cells then place their data values on the column bit lines by flowing a differing current, depending upon their programmed states, from a coupled source line to the coupled column bit lines.
The array of memory cells for NAND flash memory devices is also arranged such that the control gate of each memory cell of a row of the array is connected to a word-select line. However, each memory cell is not directly coupled to a column bit line by its drain region. Instead, the memory cells of the array are arranged together in strings (often termed NAND strings), typically of 32 each, with the memory cells coupled together in series, source to drain, between a source line and a column bit line. The memory array for NAND flash memory devices is then accessed by a row decoder activating a row of memory cells by selecting the word-select line coupled to a control gate of a memory cell. In addition, the word-select lines coupled to the control gates of unselected memory cells of each string are driven to operate the unselected memory cells of each string as pass transistors, so that they pass current in a manner that is unrestricted by their stored data values. Current then flows from the source line to the column bit line through each series coupled string, restricted only by the selected memory cells of each string. This places the current-encoded data values of the row of selected memory cells on the column bit lines.
The source line should have a low resistance to keep the voltage drop along the source line low so as to avoid memory device failure. Moreover, the source line should have low current leakage when biasing the source line. Source lines are often made from polysilicon, which normally satisfies current-leakage requirements, but has a relatively high resistance that can cause excessive voltage drops that may lead to device failure. Connecting each NAND string to a metal line formed on a layer different from that containing the NAND strings using a polysilicon contact can reduce resistance. However, this method requires a patterned masking step for forming the metal line and another patterned masking step for forming the contacts. Forming a metal line on a different level than the polysilicon source line and periodically connecting the source line and metal line by forming contacts between the source and metal lines can reduce the resistance compared to an all polysilicon source line. However, each contact consumes silicon real estate.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative source lines for NAND flash memory devices.